Tank Carradine had 11 sacks his final season at Florida State, and he led all 49ers defensive linemen in tackles in the preseason.

So why, with San Francisco struggling with a watered-down pass rush, is the team’s 2013 second-round draft pick standing on the sideline?

According to Carradine, his lack of playing time has nothing to do with ability but rather his mastery of the playbook, which he admits is not at the level it needs to be.

“I’m close, you know?” he said. “There are some things that I get, a lot of stuff I get. Then there’s some stuff I don’t get. And when you compare that to somebody else who’s been in the system longer, of course they’re going to know more than you.”

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That echoes defensive coordinator Vic Fangio, who nearly two months ago said Carradine has “got to do better from an assignment standpoint for us to feel comfortable to play him. Right now he’s missing too many things mentally.”

Fangio softened his message this week, saying Carradine is “improving along those lines, he really is.”

Fangio also noted that the two linemen playing ahead of Carradine – Demarcus Dobbs and Tony Jerod-Eddie – are more useful on special teams. The two have combined for 88 defensive snaps as the backups to starters Justin Smith and Ray McDonald. Dobbs has three tackles, Jerod-Eddie has two.

Neither, however, has a sack or even a quarterback pressure over that span, symptomatic of a 49ers pass rush that has suffered without outside linebacker Aldon Smith, who is suspended until mid-November.

The 49ers this week will take on an Eagles offense that is missing three of its starting offensive linemen, including center Jason Kelce, who won’t play because of a sports hernia. Still, the Eagles have allowed only five sacks this season and their passing offense averages 310 yards per game.

Meanwhile, only two 49ers – Justin Smith and Ahmad Brooks – have a sack this season, and the team is on pace to finish with 21 sacks, which would be its lowest total ever in a 16-game season.

Linebacker Corey Lemonier, who has played in place of Aldon Smith on passing downs, had a quarterback hit and pressure last Sunday but hasn’t had a sack since Week 6 of 2013.

The defense had no sacks against Arizona, despite Fangio blitzing more often – and with more defenders – than he has in two seasons. Arizona Cardinals quarterback Drew Stanton either solved the pass rush or successful blitzes were unraveled by third-down penalties.

The 49ers gave up two second-half passing touchdowns in the loss to Arizona and have allowed seven touchdowns through the air this season. Through three games, only two other teams – Philadelphia and Jacksonville – have allowed more. The 49ers’ third-down defense ranks last in the league.

“You can’t play good pass defense in this league without good pass rush,” Fangio said. “And they go hand in hand. You can’t have good pass rush without good coverage. So, we’ve had to call on it a little bit more lately here, last week. Hope to not have to play that way all the time.”

Injuries – Tight ends Vernon Davis (ankle) and Vance McDonald (knee) are listed as questionable for Sunday’s game vs. Philadelphia. So are offensive tackle Anthony Davis (hamstring) and safety Antoine Bethea (ankle), who was added to the injury report Friday.

Bethea was not walking with a limp after practice. And given that he has played in 99 straight games, it doesn’t seem likely he will let a minor injury keep him out of the 100th.

Et cetera – The investigation involving defensive end Ray McDonald is still ongoing, the Santa Clara County District Attorney’s Office said Friday. That means McDonald’s Monday court date likely will be pushed back a second time.

• Cornerback Tramaine Brock (toe) did not practice all week. Perrish Cox is likely to make his third consecutive start in Brock’s place.

About This Blog

Matt Barrows was born in Blacksburg, Va., and attended the University of Virginia. He graduated in 1995, went to Northwestern for a journalism degree a year later, and got his first job at a South Carolina daily in 1997. He joined The Sacramento Bee as a Metro reporter in 1999 and started covering the San Francisco 49ers in 2003. His favorite player of all time is Darrell Green. Reach Barrows at mbarrows@sacbee.com.
Twitter: @mattbarrows